Seasonal changes in the combined glucose-insulin tolerance test in normal aged horses

Abstract

Background: Equine metabolic syndrome (EMS) is an increasingly recognized problem in adult horses. Affected horses are often obese and predisposed to the development of laminitis, especially in the spring and summer months. In addition, in the summer and fall months, increases in endogenous insulin concentrations, a marker of EMS, have been reported.

Hypothesis/objectives: The purpose of this study was to evaluate seasonal changes in results of the combined glucose-insulin tolerance test (CGIT), a diagnostic test for EMS.

Animals: Nine healthy, aged horses with no history of laminitis and no clinical signs of EMS.

Methods: Horses were given dextrose (150 mg/kg) and insulin (0.1 U/kg) IV. Plasma glucose concentrations were measured at 0, 1, 5, 15, 30, 45, 60, 75, 90, and 150 minutes and serum insulin concentrations at 0, 5, and 75 minutes. Testing was performed in February, May, June, August, September, and November. Mean glucose concentrations, characteristics of the curve, and insulin concentrations during the CGIT were compared across months using repeated measures ANOVA (P < .05).

Results: No CGIT parameters indicated insulin resistance, but mean area under the curve for glucose concentrations was significantly lower in August and November compared to February and in November compared to June, indicating increased insulin-mediated glucose clearance. Glucose nadir was significantly lower in November compared to that in February.

Conclusions and clinical importance: No clinically relevant differences were seen in the results of the CGIT, suggesting that season minimally affects results of this test in normal aged horses in the southeastern United States.